1. Field of the Invention
The present invention relates to a fuel assembly for use in a pressurized water reactor and, more particularly, to a method of surface-treating a leaf spring provided on an upper nozzle.
2. Description of the Related Art
FIG. 4 is a diagram schematically showing a fuel assembly ordinarily used in pressurized water reactors, and FIGS. 5A and 5B are diagrams schematically showing a leaf spring. Referring to FIG. 4, the fuel assembly 1 is constituted by an upper nozzle 2, a lower nozzle 3, a plurality of grids 4, control rod guide tubes 5, and a multiplicity of fuel rods 6. More specifically, the upper nozzle 2 and the lower nozzle 3 are placed spaced apart from each other in the vertical direction, and a plurality of grids 4 are placed at certain intervals between the upper and lower nozzles 2 and 3. Each grid 4 has grid spaces formed by straps. The control rod guide tubes 5 are inserted in the grid spaces at predetermined positions and are fixed by fixing portions of the grid 4. The upper and lower ends of the control rod guide tubes 5 are connected to the upper and lower nozzles 2 and 3. The multiplicity of fuel rods 6 are elastically supported by supporting portions of the grids 4. The fuel assembly 1 is held in a core (not shown) by an upper core plate 7 and a lower core plate 8 of an in-reactor structure.
A leaf spring 9 is provided on the upper nozzle 2 by being interposed between the upper nozzle 2 and the upper core plate 7. The leaf spring 9 has the function of absorbing elongation of the fuel assembly 1 caused by irradiation as well as the differences in thermal expansion between the fuel assembly 1 and the in-reactor structure, and also has the function of preventing the fuel assembly 1 from being floated by cooling water flowing in from below.
There are certain types of fuel assembly 1 ordinarily used, e.g., the 17×17 type, the 15×15 type, and the 14×14 type selected according to the number of fuel rods 6 arrayed. FIGS. 5A and 5B illustrate the structure of an example of the 17×17 type of leaf spring 9. The leaf spring 9 is formed of an upper spring 10 and a lower spring 11. The lower spring 11 is formed of two plate members.
FIG. 5B is an enlarged detailed diagram of a portion A surrounded by the dotted line in FIG. 5A. As can be seen in FIGS. 5A and 5B, the upper spring 10 has a curved portion 12 and a vertical portion 13, and the vertical portion 13 has stepped edges 14. The lower spring 11 has a through hole 15. The upper spring 10 and the lower spring 11 are combined in an integrated structure such that the vertical portion 13 of the upper spring 10 is passed through the through hole 15, and the upper spring 10 contacts the lower spring 11 with the stepped edges 14 of the vertical portion 13.
The leaf spring 9 has a base portion 16 formed in a horizontal structure for fixation on the upper nozzle 2, that is, the base portion 16 is parallel to an upper surface of the upper nozzle 2. The leaf spring 9 is fixed on the upper nozzle 2 with the base portion 16 fastened to the upper nozzle 2 by an attachment bolt 17. The leaf spring 9 is formed so that its upper end (in the vicinity of the curved portion 12) is brought into contact with the upper core plate 7 to produce a pressing force. The leaf spring 9 is therefore bent upwardly at a comparatively large angle in the vicinity of the base portion 16. A precipitation-hardened alloy of a high proof stress (e.g., Inconel 718) is used as the material of the leaf spring 9 in order to obtain the necessary spring force.
Ordinarily, the fuel assembly 1 is used in a reactor while being immersed in a primary coolant. In the leaf spring 9, stress is caused during use in the reactor. There is a possibility of stress-corrosion cracking (SCC) caused in the leaf spring 9 by high temperature primary coolant. Also, it is thought that the state of the surface of the leaf spring material in contact with the primary coolant influences stress-corrosion cracking (SCC).
A conventional leaf spring manufacturing process under the above-described circumstances will be briefly described with reference to FIG. 6. Since as mentioned above the base portion 16 of the leaf spring 9 is fixed on the upper nozzle 2, the base portion 16 must be provided in a horizontal structure. On the other hand, the upper end of the leaf spring 9 is shaped so as to be suitable for contact with the upper core plate 7 in producing the necessary pressing force. Accordingly, bending (step 1) is performed to bend the base portion 16 of the leaf spring 9 upward. In this bending process, the leaf spring is bent so as to have a comparatively large angle. Therefore, it is necessary, from the viewpoint of workability, to perform bending before heat treatment (step 2).
However, if bending is performed before the heat treatment, there is a problem that an oxide film formed by the heat treatment attaches to the bent portion to form a Cr-deficient layer. Ordinarily, this oxide film is removed by grinding the surface. However, it is extremely difficult to remove the oxide film by grinding based on ordinary grinding methods since this grinding is performed after bending (step 1). Therefore, the bent portion is presently left in its unground state. Consequently, therefore, in addition to the work-hardened layer remaining after bending, a Cr-deficient layer is formed in the bent portion due to the oxide film. Accordingly, there is a possibility of the leaf spring 9 stress-corrosion cracking resistance characteristics (SCC resistance characteristics) deteriorating due to such layers.
As a method for surface treatment of the leaf spring 9, a method has been used in which the oxide film is removed by glass bead blasting (GBB) to obtain an improved appearance of the product after a final machining step (step 3) following the heat treatment (step 2). However, it is difficult to remove the Cr-deficient layer in the metallic portion by this GBB. Stronger GBB entails the risk of newly forming a work-hardened layer and is therefore undesirable.
The points in the above are potential factors to be a cause of stress-corrosion cracking (SCC) under a low-stress condition. That is, the Cr-deficient layer reduces the corrosion resistance of the surface, while the surface work-hardened layer increases the possibility of early cracking.